Known in the art are taps for pressing elastic thread profiles capable of tapping through holes only.
After the thread has been cut the tapped part is screwed off the tap and released over the shank of the tap.
Such a tap for pressing elastic thread profiles comprises a groove forming portion and a thread forming portion arranged on the tap axis in series and having points and threads of equal pitch. These portions are interconnected by a neck portion having a length which is less than that of the thread to be tapped. Also, the crests of the thread forming portion are axially displaced from those of the groove forming portion.
The amount of displacement is that of half the pitch of the thread to be tapped.
Motion of the groove forming portion through the hole of a part produces a helical groove either by cutting or pressing. At the moment the thread forming portion comes into operation the groove forming portion acts as a threading block providing a lead to the thread crests of the thread forming portion between the valleys of the helical groove that has been formed.
The thread forming portion impresses a thread by reshaping the helical groove into a confined space, which is arranged in the core of the thread profile that has been formed. The space reduces the sectional area of the thread profile formed, which makes it possible to increase elastic deformation of the profile along the axis of the thread.
This ensures a more favorable load distribution between threads of a threaded connection and improves its strength (SU, Inventor's Certificate No. 625,824 published Apr. 19, 1977, Int. Cl. B 21 H 3/08, of Dniepropetrovsky ordena Trudovogo Krasnogo Znameni gorny institut im. Artema).
This tap is unsuitable for pressing elastic thread profiles in blind holes because the groove forming portion, as the tap is being backed out of the hole, will distort the thread that has been formed by reason of the axial displacement of its crests from those of the thread portion.